Data from the Collaborative Perinatal Project were analyzed to determine whether moderately low birth weight, singleton babies without congenital anomalies were at increased risk of infectious disease mortality. In this study, children with birth weight of 1500 to 2499 grams were compared to children with birth weight equal to or greater than 2500 grams with regard to infectious disease mortality in the late neonatal and postneonatal periods of infancy as well as in childhood. We hypothesized that birth weight was inversely related to the risk of infectious disease mortality, with the strength of the relationship decreasing with increasing age at death and varying according to whether low birth weight was a manifestation of preterm delivery or intrauterine growth retardation.